Compost Only Goes So Far

Sometimes you need fertilizers

By Dr. Francis Gouin

Am I an organic gardener? I’m often asked that question. I suspect that many who read this column conclude that my frequent reference to composting and compost in gardening means I must be an organic gardener. They seem shocked when I say that I use chemical fertilizers in addition to amending my soils with compost. My reasoning is that of a scientist.
Before my research into compost, my primary area of research was mineral nutrition of plants. My research for the thesis for my master’s degree resulted in the development of a slow-release fertilizer marketed as Osmocote 18-6-12. The three-year study was conducted on yews, a narrow-leaf evergreen common in landscape plantings. At the time, the nursery industry was starting to grow more plants in pots. Nurserymen were using the same fertilizers as in field production. The two are totally different growing conditions. Plants growing in pots require more frequent irrigation, which washes more nutrients through the bottom of the pots. Plants grow faster in pots because of high rooting-media temperatures and better aeration. But the rooting media used for growing plants in pots do not store nutrients well.
My research and reviews of the results of many other plant-nutrient experts convinced me that it can be very difficult to satisfy the nutrient requirements of plants at different stages of growth.
My later work in the use of compost in the production of nursery and greenhouse crops reinforced the conclusion that it takes both organic and mineral fertilizers to achieve both plant health and high crop yields. In the spring when soils are cool, compost, animal manure and organic fertilizers are unable to generate sufficient nutrients because they require microbial activity to release nutrients. For spring-planted crops such as peas, cabbage, broccoli, onions and cauliflower, chemical fertilizers can provide those nutrients.
Again when tomato and pepper plants are flowering and fruiting simultaneously, the demand for nitrogen is greater than organics are capable of generating. Plants drop bottom leaves because the nitrogen within them is migrating out of the leaves and moving up the stem. This makes the plants more susceptible to blight.
In mid-summer when sweet corn is growing rapidly in advance of tasseling, organics are not capable of generating sufficient nitrogen for big, sweet ears. Thus the corn stalks drop their bottom leaves.
We know from studies on the use of compost to grow bedding plants that the nutrient-supplying power of rooting media containing compost can supply adequate levels of nutrients for only about six weeks.
Nitrogen is the element plants need in greatest abundance. Organic matter is incapable of supplying all that is needed. Numerous research studies have confirmed my initial research that most plants require five to six times more nitrogen that phosphorus and two to three times more potassium than phosphorus. As day length, moisture and temperature affect plant growth, using the combination of soils rich in organic matter with supplemental applications of chemical fertilizers gives you the best of both worlds.